Composition and Morphology of Organic and Mineral Matter in Fly Ash Derived from Bituminous Coal Combusted in the Bêdzin Power Station (Poland)

Polish Geological Institute Special Papers, 7 (2002): 237–244 Proceedings of the IV European Coal Conference

COMPOSITION AND MORPHOLOGY OF ORGANIC AND MINERAL MATTER IN FLY ASH DERIVED FROM BITUMINOUS COAL COMBUSTED IN THE BÊDZIN POWER STATION (POLAND)

Danuta SMO£KA-DANIELOWSKA1, Magdalena MISZ1

Abstract.Flyash particles formed during coal combustion are composed entirely of organic or/and mineral matter. The proportions of the two components depends on combustion conditions and the presence of minerals in feed coal particles. The aims of this paper are the classification of char morphologies, the quantification of the inert- and semiinert components, and the characterisation of the morphologies and compositions of mineral particles in fly ash from Bêdzin Power Station, Poland. Various char morphologies are presented and their distribution in individual pulverised fuel boilers is dis- cussed as are the morphologies of mineral particles and the distribution of major and minor elements in different size fractions of fly ash.

Key words:coal, pulverised fuel combustion, fly ash, chars, mineral matter.

INTRODUCTION

Coal is a complex rock containing organic and mineral mat- 1993). The product of these processes is char containing carbon ter. During the combustion process both organic and mineral and varying proportions of transformed mineral matter (Kor- matter undergo severe changes. The solid products of this dylewski ed., 1993). Volatiles undergo ignition during their process are fly ash and slag. In this paper, the interrelations release (Jarosiñski, 1996) or afterwards (Lau, Niksa, 1992). between transformed organic and mineral matter in fly ash Burnout of released volatiles in the vicinity of char causes an are described. increase in its temperature. After exceeding a critical tempera- The aims of the investigations were: ture, partly transformed coal particles undergo ignition (Cho- — the identification of the morphological forms of unburned miak, 1977) and soot may be formed (Solomon et al., 1993). organic matter, and their classification, This process takes place in the temperature range 500–1800°C — the evaluation of the total inert- and semiinert-inertinite (op. cit.). The next stage is char combustion which involves contents, 90% of the duration of the process (Kordylewski ed., 1993). — the characterisation of the morphology and mineral com- It takes place in the temperature range 900–1800°C (Solomon position of both isolated particles and their aggregates re- et al., 1993). The last stage of the process involves the disinte- sulting from coal combustion in Bêdzin Power Station gration of char particles and the formation of fly ash (op. cit.). (Poland). Pulverised fuel boilers are characterised by the highest tem- Coal combustion is a multistage process. Before coal parti- peratures of combustion, ranging from 1200–1500C, and the cles enter the combustion chamber of a pulverised-fuel boiler, shortest time of combustion. It is assumed that plasticised coal they are ground to less than 100 ìm in size. Crushed coal is de- particles do not agglomerate (Shampine et al., 1995). livered to the combustion chamber together with exhaust gases. The processes described above are influenced by such fac- During delivery, they are heated to a temperature of up to tors as the physico–chemical properties of the coal, its maceral 350C and moisture is evaporated. At higher temperatures, composition and rank on the one hand and, on the other, by ranging from 350–600C, the coal particles undergo a process temperature, rate and time of combustion. In addition, the of softening. Further increase of temperature up to 700C quantity and composition of mineral matter is important. All of causes their devolatilisation and swelling (Solomon et al., these factors influence the amount of unburned organic matter

1 University of Silesia, Department of Earth Sciences, Bêdziñska 60, 41-200 Sosnowiec, Poland 238 Danuta Smo³ka-Danielowska, Magdalena Misz in fly ash (Walsh et al., 1994) as well as the mass distribution of coals are lower than those of mineralised coals (Crelling et al., ash (Morrison, 1986). 1992; Menéndez et al., 1994). Mineral matter may also influ- Coal is a heterogeneous organic rock. It contains micro- ence fragmentation during combustion (Wigley et al., 1997). lithotypes and associated minerals. Individual coal particles Whether the various minerals and their phases undergo may be formed exclusively of one maceral or associations of changes during combustion is reflected in the composition of macerals. Their composition affects their behaviour during the solid waste products. The composition of the mineral mat- combustion. They may be divided into three groups. ter in coal and the high burnout temperatures achieved during Reactive macerals devolatilise and soften during combus- combustion in conventional power stations influence the com- tion, e.g., vitrinite and liptinite (Falcon, Snyman, 1986), and the position and mineralogy of fly ash, e.g., the occurrence of reactive portions of inertinite macerals, e.g., reactive semi- amorphous aluminosilicate glass and of high-temperature crys- fusinite, low reflective funginite, secretinite, micrinite, ma- talline phases such as mullite, quartz and magnetite (Querol et crinite and inertodetrinite (Kruszewska, 1990). al., 1996). These products are the result of various processes Inert macerals such as fusinite (pyrofusinite) and semi- such as decomposition, oxidation, reduction, dehydration, fusinite (pyrosemifusinite) do not undergo changes during hydration, carbonatization and sulphatization, melting and so- combustion (op. cit.). lution. In addition, pre-existing and newly-formed compounds Semiinert macerals exhibit properties intermediate be- also react with one another. tween reactive- and inert macerals during combustion. These processes result in the formation of fly ash particles Though some coal particles may consist entirely of or- that provide a matrix for the interactions of a great variety of ganic or mineral matter, most involve different proportions of substances formed during coal combustion and the emission these two constituents. The most common mineral groups process. The size of the fly ash particles depends on several occurring in coal are clay minerals, sulphides, carbonates and factors, e.g., initial coal particle diameters, mineral content and silica (Stach et al., 1982). Mineral matter can occur in syn- and its distribution within coal particles, degree of coalescence of epigenetic forms. In many cases, mineral matter co-exists fused mineral matter and number and size of fragments pro- with macerals as carbominerites. It can be dispersed within duced during combustion (Morrison, 1986). macerals as single particles a few m in size or as aggrega- Fly ash particles can be classified with regard to chemical tions of such particles. The shape of these aggregates can be composition and origin into coal particles, combustion particles spherical, oval or lenticular. Mineral matter can also fill and condensation particles (Querol et al., 1995). The latter intercellular spaces. form in the high temperatures of the combustion chamber when Though it is a source of air pollution, inorganic matter in the largest mineral particles fuse and then condense as cenosp- coal does confer some advantages — and disadvantages — heres (Karr, 1979). It is likely that the spherical form reflects during the combustion process. Coal particles containing mi- the action of surface tension (Wadge et al., 1986). neral matter are characterised by a higher specific heat capa- city. They need more time to be combusted. Mineral matter Five pulverised-fuel boilers are installed in Bêdzin Power may also act as catalyser (Wigley et al., 1997). Chars containing Station. Two steam boilers of OP-120 type (K-6 and K-7), one greater amounts of calcium and magnesium impurities are is a water boiler of WP-70 type (K-5) and two are water boilers more reactive due, probably, to the catalytic activity of these of WP-120 type (K-8 and K-9) (Materials, 1995). The thermal elements during coal oxidation (Walker et al., 1968 in Jenkins efficiency of the boilers ranges from 88–90% (E. Piotrowska, et al., 1973). Coal demineralisation may either cause a de- oral inf.). The exhaust gases are carried away through electro- crease in coal reactivity or an increase. Any increased reacti- filters to a single chimney. Every year, 279.1 thousand tons of vity is probably the result of an increase in porosity enhancing coal are combusted in these boilers (Materials, 1995). The heat- oxygen migration into the combusting coal particles (Jenkins ing value of the feed coal is 21.2 MJ/kg, the ash content ranges et al., 1973). The combustion temperatures of demineralised up to 20% and sulphur content up to 0.8%.

SAMPLE COLLECTION AND ANALYSIS

Samples of fly ash were collected once a week during the The following analyses were carried out: period November 6, 1996–June 26, 1997. In total, 77 fly ash — analysis of unburned organic matter contents, samples were collected from five pulverised fuel boilers — — determination of char morphological forms at 500 points 5 samples from WP-70 (K-5), 27 from OP-140 (K-6), for each sample, 26 from OP-140 (K-7), 9 from WP-120 (K-8), and 10 from — evaluation of the total-, inert- and semiinert inertinite con- WP-120 (K-9). As the OP-140 boilers were in use during tents, most of the sampling time, most of the samples were col- — X-ray diffraction analysis, lected from these. Fly ash samples were collected at the bot- — scanning electron microscope analysis (SEMA), tom of the electrofilters. — electron microprobe analysis (EPMA). Composition and morphology of organic and mineral matter in fly ash... 239

RESULTS

Morphologies of unburned organic matter in fly ash Anisotropic networks (Plate I, Fig. 4) possibly result from the combustion of vitrinertite or trimacerite. They are always During the period of sample collection, no pattern was ob- lower in number than isotropic networks in the same sample – served in the measured unburned organic matter contents. Ex- ranging up to 11.8% only (Table 1). As with isotropic networks, cept for one sample with 18.40% unburned organic matter, all average contents of anisotropic networks are the lowest in fly of the other samples comprise between 1.33% and 12.07% of ash from the steam boilers. The determined averages from the this matter. K-5 boiler are twice as high as in the K-8 and K-9 boilers. The classification of char forms is based on porosity, wall Tenuinetworks are among the rarest forms (Table 1). thickness, presence/absence of anisotropy and the degree of These may originate from clarite with a liptinite content higher alteration. That used here is a modified classification based on than that of vitrinite. In many samples, including all from the classifications given by Jones et al. (1985), Tsai, Scaroni K-8 boiler, this form is absent. The highest quantities (1.2%) (1987), Oka et al. (1987), Bailey et al. (1990), Bend et al. were seen in fly ash from the K-5 and K-6 boilers and the highest (1992), Rosenberg et al. (1996), ICCP (2001). average content (0.3%) from the K-5 boiler. The following morphological forms of unburned organic Honeycombs (Plate I, Fig. 5), probably resulting from matter were determined in the fly ash samples (Misz, 1999, combustion of semiinert inertinite, make up to 9.6% of all char 2002): forms in the fly ash. Fly ash from the K-6 and K-7 boilers con- Crassispheres (Plate I, Fig. 1), one of the most frequent tains the least amounts and fly ash from the K-5 boiler the forms in fly ash, derive from vitrinite. Their contents in the ana- greatest amounts (Table 1). lysed fly ash samples range from 0.6–37.0%. The highest aver- Inertinite (Plate I, Figs. 6, 7) forms from the most inert part age contents of crassispheres were observed in fly ash from the of the feed–coal inertinite, particularly from pyrofusinite, pyro- K-6, and K-7 boilers (20%). Lower average contents were ob- semifusinite and in part from secretinite, funginite and the most served in fly ash from the K-5 boiler (17.4 %) and the lowest inert part of inertodetrinite. Some fly ash inertinite is morp- average contents were measured from the K-9 (10%) and K-8 hologically identical to that in the feed coal. The contents of this (12%) boilers (Table 1). unaltered inertinite range up to 8.6% (Table 2). Fly ash from K-5, Tenuispheres (Plate I, Fig. 2) probably derive from cla- K-6 and K-7 boilers reveal similar average amounts (4%). Fly rite. Their contents (up to 5.0%) are always significantly ash from the K-5 boiler has a higher average content of unaltered lower than the crassisphere contents in the same sample of fly inertinite; boiler operation conditions are probably a key factor. ash. The fly ash from all of the boilers reveal average tenuisp- Slightly altered parts of inertinite in both slag and fly ash here contents of about 1.6%. Boiler K-5 (2.9%) is somewhat show small, round pores, irregular cracks and rounded edges. exceptional (Table 1). This morphological subgroup is less common than unaltered Isotropic networks (Plate I, Fig. 3) are common in the fly inertinite. As with unaltered inertinite, the highest average con- ash examined (Table 1). They probably originate from cla- tent of slightly altered inertinite is found in fly ash from the K-5 rodurite and duroclarite or from some forms of vitrinertite. boiler (Table 2). Their quantities in fly ash range from 4.0–44.2% and are lowest Detritus (Plate I, Fig. 8) is the dominant form of char in the fly in fly ash from the K-6 and K-7 boilers. Fly ash from the smallest ash. Contents range from 12.6–85.8% . The highest detritus con- boiler (K-5) revealed the highest average contents (27.2%). tents, and the highest average contents, characterise the K-8

Table 1 Contents of char morphological forms in fly ash from pulverised fuel boilers at Bêdzin Power Station [in %]

Isotropic Anisotropic Tenuinet- Crassispheres Tenuispheres Honeycombs Inertinite Detritus networks networks works

min. 9.4 0.6 13.8 2.4 0.0 1.0 7.4 15.2 K-5 max. 23.6 4.8 37.4 11.8 1.2 9.6 12.8 56.2 average 17.4 2.9 27.2 5.0 0.3 3.4 9.6 34.2 min. 10.8 0.0 7.4 0.0 0.0 0.0 1.2 16.0 K-6 max. 37.0 5.0 36.4 6.0 1.2 5.0 14.6 76.6 Average 20.5 1.6 20.2 1.6 0.1 1.7 5.8 48.6 min. 9.4 0.0 4.0 0.0 0.0 0.0 1.0 12.6 K-7 max. 33.0 5.0 37.4 5.2 0.4 5.0 12.2 82.6 average 19.5 1.6 17.8 1.7 0.0 1.8 4.7 52.8 min. 8.0 0.0 6.2 0.2 0.0 0.0 2.2 35.6 K-8 max. 14.6 4.0 35.4 6.2 0.0 5.8 8.4 78.8 average 12.0 1.7 19.5 2.3 0.0 1.9 4.8 57.8 min. 0.6 0.2 8.4 0.0 0.0 0.2 1.4 29.0 K-9 max. 26.8 3.4 44.2 8.2 0.2 4.6 12.4 85.8 average 10.0 1.5 21.4 2.6 0.0 2.0 5.7 56.8 240 Danuta Smo³ka-Danielowska, Magdalena Misz

Table 2 many pores filled with oxides of potassium, sodium, calcium, Inertinite contents in fly ash samples from boilers magnesium, iron, titanium, manganese, sulphur, phosphor and in Bêdzin Power Station [in %] barium (Plate II, Fig. 3). Inertinite K-5 K-6 K-7 K-8 K-9 Cereda et al. (1995) classified aluminosilicate particles on the basis of two factors. The first is the most important. High min. 7.4 1.2 1.0 2.2 1.4 and positive factor loadings for Al and Si and high negative fac- Total max. 12.8 14.6 12.2 8.4 12.4 tor loadings for all other elements confirm a connection between average 9.6 5.8 4.7 4.8 5.7 enriched concentrations of minor and trace elements in parti- min. 4.8 1.0 1.0 1.8 1.2 cles and lower concentrations of Al and Si and accounts for Unaltered max. 8.4 8.6 8.0 6.8 8.2 69.9% of the variance. The second factor, accounting for average 6.2 4.2 3.3 3.5 3.9 16.4% of the variance, is related to the variability in the distri- min. 2.2 0.0 0.0 0.4 0.2 bution of Ca concentrations. The occurrence of different parti- Slightly max. 4.4 6.0 1.6 1.6 7.2 altered cle classes reflects the heterogeneous nature of the mineral average 3.5 1.5 1.1 1.1 2.1 matter in the parent coal and combustion-related, particle- -formation mechanisms. Coal combustion is one of the major sources of atmospheric and K-9 boilers. Lower average contents characterise fly ash pollution by toxic trace elements. Some fly ash particles adsorb from the K-6 and K-7 boilers and the lowest average contents elements or mineral phases on their surface. Trace elements are of all were determined in samples collected from the K-5 boiler dissolved in the melts and inherited by the glass. Different ele- (Table 1). ments may also be adsorbed on the surface of fly ash particles during crystallization. Trace elements are concentrated mainly in the smallest fly ash particles. Most elements (e.g., As, Ba, Co, Cr, Mn, Transformed mineral matter in fly ash samples Ni, Pb, Zn, Cu) are concentrated in the size class below 20 m. Average trace element concentrations are given in Table 4. Inorganic mineral matter in the fly ash consists mainly of amorphous glass spheres and lesser amounts of various crystal- Table 4 line components. Major mineral phases would usually be quartz, mullite, Ca–Fe silicates, Ca–Na silicates, magnetite, Average concentrations of trace elements [ppm] in different fly ash size fractions; Bêdzin Power Station hematite, wustite, feldspars, gypsum, bassanite, lime, silli- Fraction manite, and anhydrite. Minor phases (calcite, ferrite spinel, As Ba Co Cr Mn Ni Pb Zn Cu corundum, oxides of titanium) and accessory phases (iron, apa- [m] tite) are also typical (Smo³ka, 1998). 160 10 665 16 120 418 49 18 96 300 Samples of fly ash collected from the electrostatic pre- 20 62 1720 56 160 720 185 295 916 163 cipitators were separated into fractional sizes ranging from 20–200 m. Most of the fly ash particles are smaller than 50 m (Table 3). Some of the aluminosilicate glassy particles are rich in iron. Amorphous aluminosilicate glass makes up 30–80% of the Those with a spherical shape but a porous structure may also be fly ash. Amounts differ from sample to sample. The smaller the characterised by the presence of iron oxides, mainly magnetite particle size, the greater the content of amorphous glass. Most and hematite (Plate II, Fig. 4) and, rarely, ferrite spinel and (70–92%) of the fly ash particles are < 50 m in size (Table 3). wustite. Pores occurring in the crystalline components of fly Glassy fly ash particles occur mainly as spherical forms and, ash may be filled with calcium, aluminium, silica and magne- rarely, as oval forms. Spheres in the Bêdzin fly ash have, in the sium oxides. Characteristic skeletal, dendritic and fan structu- main, diameters ranging from < 5–200 m. res result (Plate II, Fig. 5). The dendritic forms of magnetite and Aluminium and silica are the major elements in fly ash. Fol- ferrite spinel, ranging in size from 90–160 m, are derived lowing Smo³ka (1998), these glassy particles can be divided from iron-rich minerals. into two groups: smooth particles rich entirely in aluminosili- Other forms of transformed inorganic matter (mainly mag- cate (Plate II, Fig. 1) and porous particles rich in aluminosilicate netite) are represented by spherical forms, empty inside or with and other elements (Plate II, Fig. 2). Porous particles contain a gaseous bubble surrounded by a wall of inorganic matter. Magnetite, enriched in Ti, Mn and/or S, occurs as rounded Table 3 crystals and hematite as platy and lamellar crystals. Hematite may result from the thermal transformation of, e.g., pyrite Size distribution of particles in fly ash; Bêdzin Power Station present in the feed coal. Larger fly ash particles tend to have Fraction [m] Lowest [%] Highest [%] Average [%] lower hematite contents. The incorporation of Fe into the glass phase suggests that 200–160 0.10 3.90 2.10 the melt exceeded temperatures of 1300° C. This would be con- 160–90 0.40 4.60 2.60 sistent with the temperature of the furnace. The abundance of 90–50 1.20 15.60 7.30 magnetite, hematite and ferrite spinel indicates that the glass 50–20 5.50 22.10 16.40 was formed under oxidizing conditions with excess air (Muk-  20 55.80 91.20 71.60 hopadhyay et al., 1996). Composition and morphology of organic and mineral matter in fly ash... 241

CONCLUSIONS

Our observations outlined above show that, in Bêdzin Fly ash particles, released into the environment in exhaust Power Station, boilers of similar type and size produce simi- gases as a result of coal combustion, provide a matrix for the lar average contents of given char morphological forms. The interaction of a great variety of substances during the combus- average quantity of tenuispheres, isotropic and anisotropic tion and emission processes. The major elements involved are networks, tenuinetworks, honeycombs and inertinite is gene- aluminium, silica, iron, calcium, sulphur, and titanium. rally higher in the WP-70 boiler than in other boiler types. Small particles of fly ash may be transported long distances The lowest average contents of detritus and unburned or- from the source of their emission. Fly ash particles inhaled and ganic matter also characterise this boiler type. This pattern deposited in the lung can cause serious health problems. That is of occurrence of the different char morphological forms is one reason why knowledge of fly-ash properties, trace element probably influenced by the fact that the WP-70 has the lowest contents, and the chemical interactions that take place in boilers boiler load demand, and the shortest operating times in hours must continuously be improved in a world where coal will be per day. used for a considerable time to come.

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PLATE I

Figures 1–8. Magnification 200x

Figure 1. Crassisphere Figure 2. Tenuisphere Figure 3. Isotropic network Figure 4. Anisotropic network Figure 5. Honeycomb Figure 6. Inertinite Figure 7. Inertinite Figure 8. Detritus Pol. Geol. Inst. Special Papers 7, 2002 PLATE 1

Figure 1 Figure 2

Figure 3 Figure 4

Figure 5 Figure 6

Figure 7 Figure 8

Danuta SMOLKA-DANfELOWSKA, Magdalena MlSZ- Composition and morphology of organic and mineral matter in fly ash derived from bituminous coal combustcd in Bf\:dzinPower Station (poland) PLATE II Pol. Geo!. Inst. Special Papers 7, 2002

Figure 1 'Figure 2

Figure 3 Figure 4

Figure 5

Figure I. SEM. Smooth particle of aluminosilicate Figure 2. SEM. Porous particle of alwninosilicate Figure 3. SEM. Particle xomposed of AI, Si, Ba, P oxides Figure 4. Magnetite particle. Magnification 600x Figure 5. Dendritic structure of iron oxides

Danuta SMOLKA·DANJELOWSKA, Magdalena MlSZ- Composition and morpho logy of organic and mi.nl!rai matter in fl y ash derived from bituminous coal combusted in B~dzin PowerStation(Poland )